Since the adoption of the Advanced Television Systems Committee (ATSC) digital television (DTV) standard in 1996, there has been an ongoing effort to improve the design of receivers built for the ATSC DTV signal. The primary obstacle that faces designers in designing receivers so that they achieve good reception is the presence of multipath interference in the broadcast television channel.
The broadcast television channel is a relatively severe multipath environment due to a variety of conditions that are encountered in the channel and at the receiver. Strong interfering signals may arrive at the receiver both before and after the largest amplitude signal. In addition, the signal transmitted through the channel is subject to time varying channel conditions due to the movement of the transmitter and signal reflectors, airplane flutter, and, for indoor reception, people walking around the room. If mobile reception is desired, movement of the receiver must also be considered. Designers add equalizers to receivers in order to cancel the effects of multipath interference and thereby improve signal reception.
Because the channel is not known a priori at the receiver, the equalizer must be able to adapt its response to the channel conditions that it encounters and to changes in those channel conditions. To aid in the convergence of an adaptive equalizer to the channel conditions, the field sync segment of the frame as defined in the ATSC standard may be used as a training sequence for the equalizer.
The frame as defined in the ATSC standard is shown in FIG. 1. Each frame contains two data fields, each data field contains 313 segments, and each segment contains 832 symbols. The first four of these symbols in each segment are segment sync symbols having the predefined symbol sequence [+5, −5, −5, +5].
The first segment in each field is a field sync segment. As shown in FIG. 2, the field sync segment comprises the four segment sync symbols discussed above followed by a pseudo-noise sequence having a length of 511 symbols (PN511) followed in turn by three pseudo-noise sequences each having a length of 63 symbols (PN63). Like the segment sync symbols, all four of the pseudo-noise sequences are composed of symbols from the predefined symbol set {+5, −5}. In alternate fields, the three PN63 sequences are identical; in the remaining fields, the center PN63 sequence is inverted. The pseudo-noise sequences are followed by 128 symbols, which are composed of various mode, reserved, and precode symbols. The next 312 segments of the field are each comprised of the four segment sync symbols followed by 828 8 level symbols that have been encoded with a 12 phase trellis coder.
Because the first 704 symbols of each field sync segment are known, these symbols, as discussed above, may be used as a training sequence for an adaptive equalizer. The original Grand Alliance receiver used an adaptive decision feedback equalizer (DFE) with 256 taps. The adaptive decision feedback equalizer was adapted to the channel using a standard least mean square (LMS) algorithm, and was trained with the field sync segment of the transmitted frame.
However, because the field sync segment is transmitted relatively infrequently (about every 260,000 symbols), the total convergence time of this equalizer is quite long if the equalizer adapts only on training symbols prior to convergence. Therefore, it is known to use the symbol decisions made by the receiver in order to adapt equalizers to follow channel variations that occur between training sequences.
An adaptive decision feedback equalizer in an 8 VSB receiver would be expected to use an 8 level slicer to make the symbol decisions that would be used to adapt the equalizer to the channel between transmissions of the training sequence. However, use of a symbol slicer results in many symbol decision errors being fed to the feedback filter of the decision feedback equalizer when the channel has significant multipath distortion or a low signal to noise ratio. These errors give rise to further errors resulting in what is called error propagation within the decision feedback equalizer. Error propagation greatly degrades the performance of the decision feedback equalizer.
The present invention instead relies on decoders to avoid the convergence and tracking problems of previous decision feedback equalizers.